The transformer core is ferromagnetic
in order to "transport" a large magnetic flux B
produced by the primary coil to the secondary coil. What I want is that the
induced flux B follows the primary field H as
closely as possible.

In other words: There should be
no hystereses loop - just a straight line,
as shown below

The ideal curve, without any
hystereses, does not exist. What you get is something like the curve shown for
a real soft magnet - because that is what we
call a material with a kind of slender hystereses curve and thus small values
of coercivity and
remanence

If we switch on a positive field H
and then go back to zero again, a little bit of magnetization is left. For a
rather small reverse field, the magnetic flux reverses, too - the flux
B follows H rather closely, if not exactly.

Hystereses losses are small, because
the area enclosed in the hystereses loop is small.

But some
losses remain, and the "transformer core" industry will be very happy
if you can come up with a material that is just 1 % or
2 % "softer" than what they have now.

Beside losses, you have another problem: If you
vary H sinusoidally, the output will be a somewhat distorted
sinus, because B does not follow H linearly. This
may be a problem when transforming signals.

A soft magnetic material will
obviously not make a good permanent
magnet, because its remaining magnetization (its remanence) after
switching off the magnetic field H is small.

But a permanent magnet is what we
want for a magnetic storage
material. Here we want to induce a large permanent magnetization by
some external field (produced by the "writing head" of our storage
device) that stays intact for many years if needs be. Some more information
about magnetic storage can be found in an extra module

It should be strong enough - even so it is contained in a tiny
area of the magnetic material on the tape or the storage disc - to produce a
measurable effect if the reading head moves over it. It should not be too strong, however, because that would make it too
difficult to erase it if we want to overwrite it with something else. In short,
it should look like this

We can define what we want in terms
of coercivity and remance. Ideally, the hystereses curve is very
"square.

At some minimum field, the magnetization is
rather large and does not change much anymore.

If we reverse the field direction, not much
happens for a while, but as soon as we move above slightly above the coercivity
value, the magnetization switches direction completely.

Ferromagnetic losses are unavoidable, we simply
must live with them

Pretty much all possible applications
- consult the list in the next section - either calls for soft or for hard
magnets; there isn't much in between.

The question was: What is available in terms of
hystereses curves? Good question; it
immedately provokes another questions:

What is available in terms of
ferromagnetic materials? The kind of
hystereses behavior you get is first of all a property of the specific material
you are looking at.

For arbitrary chemical compounds,
there is little predictive power if they are ferromagnetic or not. In fact, the
rather safe bet is that some compound not
containing Fe, Ni, or Co is not ferromagnetic.

Even if we restrict ourselves to some
compound or alloy containing at least one of the ferromagnetic elements
Fe, Ni or Co, it is hard to predict if the result will be
ferromagnetic and even harder to predict the kind of hystereses curve it will
have. Pure Fe in its (high temperature) fcc lattice variant is
not magnetic, neither are most variants of stainless steel, for example.

But progress has been made -
triggered by an increasing theoretical understanding (there are theories, after
all), lots of experience and semi-theoretical guide lines - and just plain old
trying out in the lab.

This is best
demonstrated by looking at the "strength" of permanent magnets as it
went up over the years:

The final question was: Can I change
the hystereses curve of a given material in a defined direction?

The answer is: Yes, you can - within
limits, of course.

The hystereses curve
results from the relative ease or difficulty of moving domain walls in a given
material. And since domain walls interact with stress and strain in a material, their
movement depends on the internal structure of the material; on the kind and
density of crystal lattice defects.

This is best illustrated by looking
at hystereses curves of one and the same
material with different internal structures.

There is a big
difference for annealed, i.e. relatively defect free iron and heavily deformed
iron, i.e. iron full of dislocations, as the figure on the left nicely
illustrates

We will find similar behavior for most
ferromagnetic materials (not for all, however, because some are
amorphous).

Instead of manipulating the defects in the
materials to see what kind of effect we get, we can simply put it under
mechanical stress, e.g. by pulling at it. This also may change the hystereses
curve very much:

Here we have the hystereses curves of
pure Ni samples with and without mechanical tension. The effects are
quite remarkable

In this case the tension force was
parallel to the external field
H

In this case the tension force was at right angles to the external field
H

There is a big change in the remanence, but not
so much difference in the coercivity.

Big changes in the remanence, not so much effect
in the coercivity. We have an almost box-like shape, coming close to the ideal
hard magnet from above.

The final word thus is:

There is a
plethora of ways to design
ferromagnetic properties out there. The trouble is, we are just learning now
how to do it a little bit better than by pure trial and error.

The future of magnetism looks bright.
With an increased level of understanding, new materials with better properties
will result for almost sure. Time will tell.